Regenerative air preheaters are used on large fossil fuel boilers to preheat the incoming combustion air from exiting hot exhaust gases. These recycle energy and conserve fuel. Recovering useful heat energy that would otherwise be lost to the atmosphere is an effective way to gain significant cost savings, conserve fossil fuels, and reduce emissions.
One type of regenerative heat exchanger, a rotary regenerative heat exchanger, is commonly used in fossil fuel boilers and steam generators. Rotary regenerative heat exchangers have a rotor mounted in a housing that defines a flue gas inlet duct and a flue gas outlet duct for the flow of heated flue gases through the heat exchanger. The housing further defines another set of inlet ducts and outlet ducts for the flow of gas streams that receive the recovered heat energy. The rotor has radial partitions or diaphragms defining compartments between the partitions for supporting baskets or frames to hold heating elements that are typically heat transfer sheets. Referring to FIG. 1, a rotary regenerative heat exchanger, generally designated by the reference number 10, has a rotor 12 mounted in a housing 14.
The heat transfer sheets are stacked in the baskets or frames. Typically, a plurality of sheets are stacked in each basket or frame. The sheets are closely stacked in spaced relationship within the basket or frame to define passageways between the sheets for the flow of gases. Examples of heat transfer element sheets are provided U.S. Pat. Nos. 2,596,642; 2,940,736; 4,363,222; 4,396,058; 4,744,410; 4,553,458; 6,019,160; and 5,836,379.
Pending U.S. patent application (WO5/006-0) Ser. No. 12/437,914 filed May 8, 2009 entitled “Heat Transfer Sheet For Rotary Regenerative Heat Exchanger”, published Nov. 11, 2010 describes different designs for heat exchange sheets, hereby incorporated by reference as if set forth in its entirety herein.
Hot gases are directed through the rotary heat exchanger to transfer heat to the sheets. As the rotor rotates, the recovery gas stream (air side flow) is directed over the heated sheets, thereby causing the intake air to be heated. In many instances, the intake air is provided to the boiler for combustion of the fossil fuels. Hereinafter, the recovery gas stream shall be referred to as combustion air or input air. In other forms of rotary regenerative heat exchangers, the sheets are stationary and the flue gas and the recovery gas ducts are rotated.
Current designs of heat transfer sheets only recover a portion of the heat in the exhaust flue gases with the unrecovered heat passing out of the stack as waste energy. The more efficiently these heat transfer sheets operate, the less the wasted heat.
Currently, there is a need for more efficient heat exchange sheet designs.